Pressure-sensitive adhesives each having various properties have been used in industrial applications. However, materials for most of the adhesives are viscoelastic bodies each subjected to flexible bulk designing. Because of its low modulus, a pressure-sensitive adhesive formed of a viscoelastic body becomes wet to conform to an adherend, thereby expressing its adhesive strength.
Meanwhile, columnar fibrous structures each having a fine diameter as novel pressure-sensitive adhesives have been known to show adhesive properties. It has been elucidated that the structures each follow the surface unevenness of an adherend to express its adhesive strength by virtue of a van der Waals force because the structures each have a diameter of the order of 10−6 m to 10−9 m.
A method of using the columnar fibrous structures each having a fine diameter in a pressure-sensitive adhesive is, for example, (1) a technology involving filling a filter having a columnar pore with a resin and removing the filter after the filling to provide the pressure-sensitive adhesive or (2) a technology involving growing the columnar fibrous structures each having a fine diameter on an Si substrate by chemical vapor deposition (CVD) to provide the pressure-sensitive adhesive (Patent Documents 1 to 3).
However, the above-mentioned technology (1) involves the following problem. That is, a filter that can be used is limited, and hence a columnar fibrous structure that can be produced has an insufficient length and a low adhesive strength.
In addition, in the above-mentioned technology (2), the adhesive strength of any one of the columnar fibrous structures is high, and obtains a value equivalent to that of a general-purpose pressure-sensitive adhesive in terms of an adhesive strength per unit area. However, the technology involves the following problem. That is, when evaluation for adhesive strength is performed in an adhesion area of about 1 cm2 in accordance with an adhesion evaluation method for a pressure-sensitive adhesive which has been generally performed (Patent Document 3), the shear adhesive strength of the columnar fibrous structures is low, and is weak as compared with that of a conventional general-purpose pressure-sensitive adhesive.
In addition, properties requested of pressure-sensitive adhesives vary depending on applications. Of those, heat resistance is needed for a pressure-sensitive adhesive to be used under a high-temperature condition. However, pressure-sensitive adhesives using an acrylic resin, a rubber-based resin, a styrene-butadiene copolymer-based resin, and the like as raw materials serving as general-purpose pressure-sensitive adhesives that have been generally used each involve the following problem. That is, the pressure-sensitive adhesives decompose at temperatures equal to or more than 200° C. because those resins each have a low decomposition temperature. In addition, even a pressure-sensitive adhesive using a raw material except such resins as described above involves a large change in modulus under a high-temperature condition as compared with its modulus at room temperature. Accordingly, such a problem that the adhesive strength of the pressure-sensitive adhesive under the condition pales beside that at room temperature, or a contamination problem due to an adhesive residue or the like arises.
In addition, a pressure-sensitive adhesive to be repeatedly bonded to or peeled from a plurality of adherends is requested to be free of adherend selectivity. However, the pressure-sensitive adhesives using an acrylic resin, a rubber-based resin, a styrene-butadiene copolymer-based resin, and the like as raw materials serving as general-purpose pressure-sensitive adhesives that have been generally used each involve the following problem. That is, the adhesive strength of any such resin depends on the surface free energy of an adherend, and hence the pressure-sensitive adhesives each show adhesive strengths largely different from each other for adherends largely different from each other in surface free energy.    Patent Document 1: U.S. Pat. No. 6,737,160 A    Patent Document 2: US 2004/0071870 A1    Patent Document 3: US 2006/0068195 A1